Conventionally, domestic access to the Internet has taken place through the PSTN (Packet Switched Telephone Network), by using analogue modems. Such access is characterised by a rather limited latency between the page request and the page presentation on the user's equipment.
With such kind of access, download time of a web page depends, in a first approximation, only on the ratio between the page size (in bytes) and the channel throughput. Thus, most web pages currently available have been designed so as to ensure limited download times through a reduction of their overall sizes.
With the increasing use of wireless techniques, such as those conforming to GPRS (General Packet Radio Service), EDGE (Enhanced Data rates for GSM and IS-136 Evolution) and UMTS (Universal Mobile Telecommunication System) standards, for access to the Internet, the criterion of reducing the overall page sizes is no longer adequate, due to the very high latency time that characterises mobile wireless networks.
In a high latency network, the download time of a web page depends not only on the page size, but also on the number of objects referenced therein. In fact, a HTTP (Hyper Text Transfer Protocol) negotiation is necessary for downloading each object, and such negotiation needs a minimum time corresponding to the round trip time (RTT) of the network, which time is the sum of the network latencies in both directions.
Consequently, for a same nominal throughput of a PSTN and a wireless link (e.g. a GPRS link), access to and download of a web page through a GPRS link is much slower than through a PSTN modem.
A number of products, generally designed as Performance Enhancing Proxies (PEP), intended to decrease web page download time in high-latency networks, have become commercially available and are described in the literature. The general principles of the PEPs and of their application can be found in document RFC 3135 of The Internet Society, “Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations”, by J. Border et al., which document is available at the site http://www.ietf.org/rfc3135.txt.
Essentially, two classes of PEPs exist: client-server PEPs, demanding installation of a software on the client device, and client-free PEPs, which do not have such requirement.
An example of client-free PEP is disclosed in WO 03/15330 A, which teaches a parallelisation of a number of HTTP requests and, consequently, of the objects being downloaded. A data compression is further performed before forwarding the images to the user.
The client-server architectures are generally based on replacing the standard HTTP protocol by more performant protocols, and thus they are intrinsically more efficient.
Examples are disclosed in WO 01/63420 A, which teaches a system where use of predictive requests and a predictive server is made, or in US 2003/197725 A and U.S. Pat. No. 6,704,024 B, which disclose systems where web pages and other visual contents are rasterised and displayed on the client device as bitmap images.
US 2001/003823 A discloses a system, based on a client-server architecture, for downloading a web page in a manner suitable for display on a television screen. This document discloses a web page conversion aimed, inter alia, at reducing the latency time. To this end, such system separates the text and image portions of an original page by downloading first the text portion of the page with any image replaced by a corresponding placeholder; then the images are retrieved and downloaded, in order to fill the placeholders. Applicants remark that, due to the use of a client-server architecture, this system has the general drawbacks inherent in this type of architecture. Moreover, downloading the text and the images in subsequent phases reduces only the apparent time of the page download, since this procedure is based on the assumption that the latency is due to the time needed by the server to provide the images themselves. The actual latency time, related to the number of HTTP negotiations, is not however reduced by such systems, since the images are individually downloaded. Moreover displaying first the text only, and then the whole page with the images, can be uncomfortable for the user.